In a typical fluidized-bed polymerization system, monomer (and possibly co-monomer, liquid diluents, and/or catalyst and possibly co-catalyst) is fed into one or more reactors. The monomer (and possibly co-monomer) reacts to produce a product effluent containing polymer particles of various sizes enriched with dissolved diluents (if used), unreacted gaseous monomer, unreacted gaseous co-monomer (if used), catalyst and co-catalyst (if used). The effluent is removed from the reactor, and typically contains between about 2 to about 20 weight % gases, between about 0 to about 10 weight % liquids, and between about 70 to about 98 weight % solids. For economical operation of this process, the unreacted monomer (and possibly co-monomer and/or diluent) is typically separated from the polymer particles and then returned to the reactor(s).
As used in the industry, the term “polymer particle(s)” typically includes solid polymer particles and/or polymer particles that are enriched with dissolved diluents.
Conventional methods of making polymer particles and methods of separating the polymer particles from fluids are generally disclosed in, inter alia, U.S. Pat. Nos. 4,543,399; 4,588,790; 5,028,670; 5,317,036; 5,352,749; 5,405,922; 5,436,304; 5,453,471; 5,462,999; 5,616,661; and 5,668,228; all of which are fully incorporated herein by reference.
For example, U.S. Pat. No. 5,028,670, assigned to BP Chemicals Limited, discloses a process and apparatus for the gas-phase polymerization of olefins in a fluidized-bed reactor maintained at a temperature T1. A gaseous reaction mixture comprising olefins to be polymerized passes through the reactor and is recycled to the reactor by means of a recycling line comprising successively a first heat transfer means, a compressor, and a second heat transfer means. A readily volatile liquid hydrocarbon is introduced into the inlet of the first heat transfer means or into the recycling line, upstream and in the vicinity of the first heat transfer means. The first heat transfer means cools the gaseous reaction mixture to a temperature T2, below T1, while volatilizing the readily volatile hydrocarbon and without condensing a constituent of the gaseous reaction mixture. The second heat transfer means cools the gaseous reaction mixture to a temperature T3, below T2, in order to maintain the fluidized-bed at the desired temperature T1.
In another example, U.S. Pat. No. 5,436,304, assigned to Exxon Chemical Patent Inc., discloses polymerizing or copolymerizing alpha-olefins either alone or in combination with one or more other alpha-olefins in a gas phase reactor having a fluidized bed and a fluidizing medium such that the fluidizing medium entering the reactor comprises a gas and a liquid phase. During reactor operation, the product is removed from the reactor through a discharge system. The discharge of polymer product is preferably followed by separation of fluids from the polymer product. These fluids may be returned to the recycle stream line as a gas and/or as a condensed liquid.
In a still further example, U.S. Pat. No. 4,543,399, assigned to Union Carbide Corporation, discloses a process for increasing the space time yield of polymer production in a fluidized bed reactor employing an exothermic polymerization reactor by cooling the recycle stream to below its dew point and returning the resultant two-phase fluid stream to the reactor to maintain the fluidized bed at a desired temperature above the dew point of the recycle stream. On discharge of particulate polymer product from the reactor, it is desirable and preferable to separate fluid from the product and to return the fluid to a recycle line. In one such system, fluid and product leave the reactor and enter a product discharge tank. Positioned above and below the product discharge tank are at least two conventional valves, one being adapted to provide passage of product into a product surge tank. The other valve releases fluid to second surge tank. Fluid from the second surge tank is directed through a filter absorber and then through a compressor and into the recycle line.
Additional references of interest include: U.S. Pat. No. 4,543,399 to Jenkins et al.; U.S. Pat. No. 4,588,790 to Jenkins et al.; U.S. Pat. No. 5,028,670 to Chinh; U.S. Pat. No. 5,317,036 to Brady et al.; U.S. Pat. No. 5,352,749 to DeChellis; U.S. Pat. No. 5,405,922 to DeChellis; U.S. Pat. No. 5,436,304 to Griffin; U.S. Pat. No. 5,453,471 to Bernier; U.S. Pat. No. 5,462,999 to Griffin; U.S. Pat. No. 5,616,661 to Eisinger; U.S. Pat. No. 5,668,228 to Chinh; and U.S. Pat. No. 6,910,343 to Ozaki.
The use of a mechanical compressor(s) to aid in the recycling of unreacted monomer from the product tanks to the reactor is generally undesirable in the foregoing processes, as these mechanical compressors have relatively high capital expense and operating costs. Additionally, the pressure in the tanks is generally close to the reactor pressure for a significant amount of the discharged gases to flow unaided from the product tanks to the reactor. Accordingly, there is a need for a process that can separate polymer particles from fluids and recycle, without mechanical compression, a significant portion of the fluids. The present invention provides a solution to the aforementioned problem in that the pressure of fresh monomer delivered to the reactor is generally higher than what is necessary to run the reactor. Accordingly, high pressure monomer may be used as the motive fluid to an ejector to create low pressure in the product tank(s). The lower pressure in the tanks would eject gases, cause dissolved fluid in the polymer product to evolve and suck the free and evolved gases to the throat of one or more ejectors. The mixed stream of motive monomer and ejected monomer may be fed to the reactor without additional compression. In this manner, less energy is required to recycle monomer from the product tanks to the reactor than would be used if a compressor system were used to recycle the monomer.